High intensity light sources, and particularly white high intensity light sources, are interesting for various applications including spots, headlamps, stage-lighting and digital light projection. For such purposes, it is possible to make use of so-called luminescent concentrators in which shorter wavelength light is converted to longer wavelengths in a highly transparent luminescent material. Such a transparent luminescent material is illuminated by LEDs to produce longer wavelengths within the luminescent material. Converted light, which will be waveguided in the luminescent material, is extracted from a surface leading to an intensity gain or, in other words, an increase in brightness.
When using such a high intensity light source for RGB light generation, e.g. in beamers, recent developments suggest a light emitting device employing step-by-step light conversion from blue LED light to green light or via green light to red light, for instance by means of phosphor-based conversion. This requires a very small etendue, a very high luminance and a high efficiency which in turn results in the design of a very compact light source with an electrical input power of more than 10-60 W. Under such conditions substantial amounts of heat are generated, which results in adverse effects on especially the etendue and efficiency.
Such a light emitting device may be built in such a way that the components are arranged in the sequence of, as seen in the direction of propagation of the light, a blue LED, a green luminescent rod, a red phosphor and an optical lens. The red phosphor is attached, e.g. glued, between the green luminescent rod and the optical lens.
Document US 2013/0039029 A1 describes a light engine with a similar construction and built in such a way that the components are arranged in the sequence of, as seen in the direction of propagation of the light through the light emitting device, a light source, a light guide, an optical element and a luminescent element. Furthermore, an optical element is also arranged between the light source and the light guide.
In both prior art devices described above, the optical quality of the red phosphor, which should be transparent and non-scattering, and the mechanical reliability of the glue used, which should be capable of holding few elements at high temperature gradient conditions, are critical. As a result a high quality red phosphor material and high quality thermally stable glue are required, which increases the costs and limits the system reliability.
US2008/0079910A1 discloses an illumination system that includes a body containing a fluorescent material that emits light in a second wavelength range, when illuminated by light of a first wavelength range. The system further includes at least a second fluorescent material that absorbs light in at least one of the first and second wavelength ranges, and emits light in a third wavelength range. The body has an extraction area, and at least some of the light in either the second or third wavelength ranges is internally reflected within the body to the extraction area.